SSL: Add configuration option to prefer server cipher order

By default, OpenSSL (and SSL/TLS in general) lets the client cipher
order take priority.  This is OK for browsers where the ciphers were
tuned, but few PostgreSQL client libraries make the cipher order
configurable.  So it makes sense to have the cipher order in
postgresql.conf take priority over client defaults.

This patch adds the setting "ssl_prefer_server_ciphers" that can be
turned on so that server cipher order is preferred.  Per discussion,
this now defaults to on.

From: Marko Kreen <markokr@gmail.com>
Reviewed-by: Adrian Klaver <adrian.klaver@gmail.com>